2013 Capital One Cup champions Swansea City prematurely ended the defence of their trophy as Birmingham ran out 3-1 winners at St Andrew's.
Goals from Dan Burn, Matt Green & Tom Adeyemi for the home side meant that Wilfried Bony's late goal was only a consolation.
Swansea, who won last season's competition by beating Bradford City in the final, made 10 changes to the team that won at Crystal Palace on Sunday, and wouldn't have expected Lee Clark's Birmingham to prove such difficult opponents.
Birmingham were without four-goal hero Jesse Lingard, with his parent club Manchester United not permitting the youngster to play in the competition. .
The Swans were the stronger team in the first-half with both Bony and Alejandro Pozuelo hitting the bar, but the teams went in 0-0 at the break. The second half, however, belonged to Birmingham.
Twelve minutes after the restart defender Dan Burn headed the home team ahead, and just four minutes later that lead had doubled.
Former Mansfield striker Matt Green beat Gerhard Tremmel at the near post, steering the ball past the Swansea goalkeeper.
The tie was all but over when Tom Adeyemi tapped home after receiving a pass from Chris Burke, who provided the creative spark for Birmingham all night.
Bony's injury-time strike for Swansea made the scoreline a little fairer for the visitors, but Blues had already built an unassailable lead.
Last week the Swans won 3-0 in Valencia, making this upset even more sweeter for Clark as his Championship side marched into the last-16.
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